This can easily be done in free roam between races. I suggest avoiding police chases, as they gain extra health and take longer to wreck. You can avoid chases altogether by taking the cop cars out in one heavy hit before a chase even begins. Hit them at speed with a head on collision or rear end them at 90+ Mph to wreck them in one hit.
Do this 5 times in one session and the achievement should pop. If you do enter a police chase just lose them and try again.